Welding Solids?

[suj]

Hi everyone,

I am a final year student, doing automotive engineering at coventry university.

In one of our courseworks, we have to design a bracket for roll cages (for a big company which i cannot state for confidential reasons)

We have designed the bracket/attachment, but have come into a dilemna.

There is a tube just say going one direction, then perpendicular (90 degree's) to it, is a rod, now we want to attach this rod by welding, but we have heard that it is hard to weld a solid, so what techniques are used to weld this type? or is it not feasible? due to being a solid not hollow?

Someone also said there might be a technique of heating up the solid to a pre-assigned temperature, and then welded? any truth behind this?

Sorry for all the questions, but myself and group mates are new to welding, so have very limited knowledge.

Any help is welcome, links, articles, journals etc

thank you in advance

Suraj

 

[GrahamGKD]

Suj,
I'm not an expert but the normal practice in these circumstances as I understand it (for any kind of arc welding) is to focus the arc more towards the solid material. The weld won't penetrate fully into the solid piece (depending on size) but then, does it need to, if it is only attached to a tubular piece? I expect the more experience people will be along shortly but I reckon they will want to know more about the dimensions of the two pieces (outside diameters and wall thickness of the tubular piece).
Good luck with the project.
Graham.

 

[piman]

Hello Suraj,

Graham has pretty well answered your question, but I am wondering why you have specified solid bar? (I'm assuming equal or close to the cage tube diameter?)

If it is to connect a dismountable member then it would be better to bore the centre so as to minimise the weight? If you can get a copy of the RAC competition licence manual (Commonly called the 'blue book') there are many drawings of roll cage detail in that, and also the relevant material specification.

Alec

 

[suj]

Suj,
I'm not an expert but the normal practice in these circumstances as I understand it (for any kind of arc welding) is to focus the arc more towards the solid material. The weld won't penetrate fully into the solid piece (depending on size) but then, does it need to, if it is only attached to a tubular piece? I expect the more experience people will be along shortly but I reckon they will want to know more about the dimensions of the two pieces (outside diameters and wall thickness of the tubular piece).
Good luck with the project.
Graham.

Hello Suraj,

Graham has pretty well answered your question, but I am wondering why you have specified solid bar? (I'm assuming equal or close to the cage tube diameter?)

If it is to connect a dismountable member then it would be better to bore the centre so as to minimise the weight? If you can get a copy of the RAC competition licence manual (Commonly called the 'blue book') there are many drawings of roll cage detail in that, and also the relevant material specification.

Alec

thank you Graham and Alec for your quick reply!

the solid bar is needed, as it is acting as a "sleeve" it isn't that long, maybe 10-15cm

as we were going to go for a hollow tube, but as it needed to have bolts to be put through it, it would need sleeves
and then tooling costs will go up, but more importantly will be alot harder to manufacture.

Weight isn't that much of an issue, as we have been told we have a fair bit of weight to play with, as it is more for "track cars" then actual competitive cars.

thanks

Suraj

 

[john224]

solid bar

Sounds to me if its for removable door bars or diagnal ,? on most cages that
I/we have built we use a piece of tube that slides in to the main pipe size used then we drill and weld in two small pipes crossways for two bolts to go through to stop the pipe crushing when tightened up,,,,

 

[suj]

Sounds to me if its for removable door bars or diagnal ,? on most cages that
I/we have built we use a piece of tube that slides in to the main pipe size used then we drill and weld in two small pipes crossways for two bolts to go through to stop the pipe crushing when tightened up,,,,

Hi John,

yes very similar, but a bit more complex, sorry if i am being so vague, it is only due to not being "allowed" to really say everything which you all can appreciate

so is arc welding the best option so far?

thanks

Suraj

 

[Luke]

roll catches are normally either MIG or TIG welded, to give the good aesthetics normally required.

Can you not provide a drawing? it doesn't have to give anything away and would provide a bit more context.

 

[suj]

roll catches are normally either MIG or TIG welded, to give the good aesthetics normally required.

Can you not provide a drawing? it doesn't have to give anything away and would provide a bit more context.

Hi Luke,

yes i can do a quick photoshop mock up just showing how it would be welded if that is ok?

thanks

Suraj

 

[malcolm]

I'm friends with a couple of track day people and they take weight very seriously. Those guys spend money on weight reduction and have a price per kg worked out. They've got price per bhp figured out too.

A modern engineer must know enough to get everyone else heading in the right direction. Marketing and the accountants in management are the people you need to convince, so you'll need to do their work too. That's why engineering is considered such a broad field and why all the good engineers get jobs in banks.

For welding you'll get the best advise after you post some dimensions. Without that info I would suspect either MIG or TIG would do the job perfectly well.

 

[suj]

I'm friends with a couple of track day people and they take weight very seriously. Those guys spend money on weight reduction and have a price per kg worked out. They've got price per bhp figured out too.

A modern engineer must know enough to get everyone else heading in the right direction. Marketing and the accountants in management are the people you need to convince, so you'll need to do their work too. That's why engineering is considered such a broad field and why all the good engineers get jobs in banks.

For welding you'll get the best advise after you post some dimensions. Without that info I would suspect either MIG or TIG would do the job perfectly well.

Hi Malcolm,

thanks for the post, we understand that, and have taken all this into concideration, i might of worded it wrong, these will not be all over the roll cage, only in a few places, so the weight addition will be small, obviously an extra 5-10kg will change power-to-weight
but we are talking an extra 200-500g (over a standard product)
we were told that safety is our number 1 priority, and lives upto the MSA Standards, a little weight will not put off the customers, so we are going by the criteria given

i will try and get some dimensions etc. done for tomorrow

thanks again to all the posts, have been very helpful

Suraj

 

[BChild]

I'm friends with a couple of track day people and they take weight very seriously. Those guys spend money on weight reduction and have a price per kg worked out. They've got price per bhp figured out too.

Losing weight is harder than gaining bhp sometimes

And I track day and given the choice between a 70 kg roll cage and one 10 kg heavier I'd take the lighter one every time

 

[Luke]

I'm friends with a couple of track day people and they take weight very seriously. Those guys spend money on weight reduction and have a price per kg worked out. They've got price per bhp figured out too.

A friend who competes in off-road continental events often says

"cheap, light, strong - pick any two"

There a plenty of other similar compromises you can come up with too.

 

[suj]

lol funny how this thread turned into a weight discussion
when we have saved weight in places where it has been optimised already lol

we are all car maniacs in our group, so we know about how much weight plays in hill-climbing/track cars
but like we said, an extra 0.5kg isn't going to be too much, as the driver can eat more then that

the only thing is our lack of knowledge for the welding, which you guys have already helped alot

i will try and get more details up tomorrow

 

[Rick]

Losing weight is harder than gaining bhp sometimes

And I track day and given the choice between a 70 kg roll cage and one 10 kg heavier I'd take the lighter one every time

Tell me about it! I've got me car down from 1040kg's to 780 and i'm really struggling now.

I've got the power from 123bhp to over 600bhp (thats the easy bit) I could easily add another 50-100hp.

lol funny how this thread turned into a weight discussion
when we have saved weight in places where it has been optimised already lol

we are all car maniacs in our group, so we know about how much weight plays in hill-climbing/track cars
but like we said, an extra 0.5kg isn't going to be too much, as the driver can eat more then that

the only thing is our lack of knowledge for the welding, which you guys have already helped alot

i will try and get more details up tomorrow

If every aspect of the car had a 0.5kg extra it would be heavy. When you serious about weight saving 0.5kgs is a LOT. I spent hours removing underseal, bits of wiring, cutting down all the bolts so theres no "overhang" to save less than 0.5kg.

As for the cage, sounds like your trying to make it easier to remove and replace?

Having the solid steel post in first then the cage sleeves over it and has a couple of bolts through it.

If this is the case you won't be able to remove the cage anyway unless its a drop top car.

 

[piman]

Hello Suraj,

regarding weight, Colin Chapman, the Lotus founder, saw a new mechanic working on one of the team cars and noticed that the mechanic had fitted washers to the fasteners. He made him remove them, he was that focussed on reducing weight no matter how small.

Alec

 

[suj]

Tell me about it! I've got me car down from 1040kg's to 780 and i'm really struggling now.

I've got the power from 123bhp to over 600bhp (thats the easy bit) I could easily add another 50-100hp.



If every aspect of the car had a 0.5kg extra it would be heavy. When you serious about weight saving 0.5kgs is a LOT. I spent hours removing underseal, bits of wiring, cutting down all the bolts so theres no "overhang" to save less than 0.5kg.

As for the cage, sounds like your trying to make it easier to remove and replace?

Having the solid steel post in first then the cage sleeves over it and has a couple of bolts through it.

If this is the case you won't be able to remove the cage anyway unless its a drop top car.

Thanks for the reply Rick,

no the weight will go up 0.5kg all together

it is not for ease of taking out, then putting back together, but more for transport (as the roll cages are made for people with no welding skills at all, who want to do everything themselves)

Hello Suraj,

regarding weight, Colin Chapman, the Lotus founder, saw a new mechanic working on one of the team cars and noticed that the mechanic had fitted washers to the fasteners. He made him remove them, he was that focussed on reducing weight no matter how small.

Alec

Hi again Alec,

that is very extreme lol, we are not selling to that range of competitiveness, more for an enthusiast

 

[suj]

H all,

the following CAD picture shows the simplified assembley of what needs welded

as you can see where the curve is, this is where it will be welded if possible




the wall thickness is 1.5mm
(the OD = 45mm, ID = 42mm)

could this wall thickness cause problems?

i hope this helps for visualisation purposes

thanks in advance

Suraj

 

[Garreth]

The wall thickness seems very thin for a roll cage - I think the usual spec is 2mm CDS tube.

I'd use a tube and sleeve it where the bolts go through - thats the way most cages are done.

Garreth.

 

[Rick]

Thanks for the reply Rick,

no the weight will go up 0.5kg all together

it is not for ease of taking out, then putting back together, but more for transport (as the roll cages are made for people with no welding skills at all, who want to do everything themselves)



Hi again Alec,

that is very extreme lol, we are not selling to that range of competitiveness, more for an enthusiast

I meant, 0.5kg's in the cage, 0.5kg on the engine mounts etc etc it soon adds up.

Theres no reason to use a solid there anyway, you can simply use a slightly thicker smaller diameter tube. I.e. Say the cage is 3mm wall thickness and 48mm, dia. Use a 42mm dia with 3.5 wall thickness (not done the calcs, just a guess) but so the strength is the same.

There going to have to weld the cage to the body anyway, while there doing that they might as well weld the door bars in as well?

 

[BChild]

Tell me about it! I've got me car down from 1040kg's to 780 and i'm really struggling now.

I started at 1650 - I'm down to 1400 fully fuelled up - I'd like to get down to 1200 kgs

I've got the power from 123bhp to over 600bhp (thats the easy bit) I could easily add another 50-100hp.

You don't need to worry about weight with 600 bhp!!! when you have 240 bhp and 1400 kgs and you can't get any more bhp every kg counts